1. Field of the Invention
The present invention relates to a pivot assembly bearing used in a hard disk drive unit, and in particular, relates to a technique which reduces the pressing force when the pivot assembly bearing is pressed into a mounting hole of a swing arm.
2. Description of Related Art
FIG. 5 is a perspective view showing a conventional hard disk drive unit D which is one example of the storage devices for a computer. In the hard disk drive unit D shown in this figure, a magnetic head 4 mounted on the tip of a swing arm 3 swingably supported by a pivot assembly bearing 1 is moved on a magnetic disk 5, whereby information is recorded on the magnetic disk 5 and recorded information is read out from the magnetic disk 5. The pivot assembly bearing 1 generally has a structure in which a pair of ball bearings is arranged between a shaft and a sleeve surrounding the shaft, and it is fitted to a mounting hole 2a provided on a base portion 2 of the swing arm 3.
However, there is a problem related to the fabrication tolerance when the pivot assembly bearing 1 is pressed into the mounting hole 2a. In the case in which the diameter size of the sleeve which corresponds to the outermost part of the pivot assembly bearing 1 approximates the upper limit of the fabrication tolerance and the bore size of the mounting hole 2a approximates the lower limit of the fabrication tolerance, the sleeve is deformed since the press-fit interference is too large, and great pressing force is required during pressing thereinto. Furthermore, metallic powder or shavings may be generated by chipping away an outer circumferential surface of the sleeve or an inner circumferential surface of the mounting hole 2a. It should be noted that the metallic powder or the shavings generated as described above are called “particles” hereinafter. The particles intervene between the sleeve and the mounting hole 2a, so that the pressing force is further increased, and this prevents smooth mounting of the pivot assembly bearing 1 to the mounting hole 2a. In addition, the particles are pushed out from the mounting hole 2a, and are held near an end surface of the sleeve, and as a result, they contaminate the hard disk drive unit D. In contrast, in the case in which the diameter size of the sleeve approximates the lower limit of the fabrication tolerance and the bore size of the mounting hole 1a approximates the upper limit of the fabrication tolerance, the pivot assembly bearing 1 may be misaligned in relation to the mounting hole 2a, since press-fit interference is insufficient. Therefore, in order to avoid the above problems only by adjusting the size, it would be necessary to set a very tight tolerance for the outer diameter of the sleeve and the inner diameter of the mounting hole 2a. 
Thus, usually, a tolerance ring for compensating the rough fabrication tolerance has been inserted in the mounting hole of the swing arm, and the pivot assembly bearing has been pressed into the tolerance ring (for example, Japanese Patent Publication No. 2004-513309 and Japanese Unexamined Patent Application Publication No. 2007-305268). The tolerance ring is a metallic member in which convex portions protruding in a radial direction are successively formed in the circumferential direction. When the pivot assembly bearing is pressed into the tolerance ring, the tolerance ring is expanded in the radial direction and the convex portions are pressed against the inner circumferential surface of the mounting hole. Consequently, the tolerance ring is elastically deformed. By this elastic deformation, the fabrication tolerance between the sleeve and the mounting hole is compensated. Moreover, as the mounting hole inner surface is pressed by the reactive force due to the elastic deformation of the convex portion, the sleeve is fixed to the mounting hole. Therefore, although the fitting size of the sleeve and the mounting hole is not strictly set, a suitable fitting condition can be obtained.
However, when the pivot assembly bearing is pressed into the tolerance ring, large pressing force is also required, and moreover, the particles are generated by chipping away the outer circumferential surface of the sleeve. Therefore, the above problems still remain, even when the tolerance ring is used.